It is well-known that a provider/operator of a communications network strives to increase the level of "survivability" of the network whenever a fault occurs, e.g., a loss of signal is detected over a communications path. The operator typically increases the level of survivability by employing a protection-switching architecture to recover from a loss of signal condition. Protection switching, as it is commonly referred to, involves establishing preassigned backup resources, e.g., a duplicated/standby communications path, which may be switched into service when a loss of signal occurs.
The detection of a loss of a signal is usually done simply by comparing the level of an optical signal received via a communications path with a predetermined threshold. If the level of the received optical signal is continuously below the threshold for a predetermined period of time, then protection switching may be requested for that communications path.
It appears, however, that the presence of one or more optical amplifiers in a communication path complicates the detection of a Loss Of Signal (LOS), since the amplifiers may not immediately reduce their output power when an LOS occurs. As a result, the level of the received signal may not change until each optical amplifier in the path has been instructed by a local controller to reduce its laser pump power and thus its output power. Disadvantageously, user data may be lost, since a substantial amount of time, e.g., 700 milliseconds, may be exhausted before the LOS is detected.
Moreover, the detection of a LOS is further complicated if the optical signal is composed of a plurality of optical signals of different wavelengths .lambda.I.